The present invention relates to off line power processing equipment and more specifically to control circuitry for dealing with momentary power line dropouts.
Modern off line power processing equipment usually contains control circuitry that senses several state variables to provide regulation and protection. Output voltage is sensed and compared to a reference to develop an error voltage used for regulation, average input voltage may be sensed to modify feedback loop gain to compensate for input voltage dependence of power train gain, and power switch current may be sensed to limit peak current in the device. Alternatively, in a circuit in which the inductor current returns to zero each cycle, volt time across the inductor may be estimated to limit the peak power switch current.
When the input voltage to the power supply disappears, the output voltage drops, causing the error voltage to rise. If input voltage gain compensation is present the feedback loop gain rises, and volt time limiting circuitry allows very long conduction intervals, since no voltage appears across the inductor. If input voltage reappears after a short interval, the control circuit may have difficulty adjusting. The high error voltage level can cause the power train to attempt to deliver a very large amount of power in a short time to raise the output voltage to the correct level. Since it may take several line cycles for the average input voltage sensing circuit to recover, loop gain may be above the designated value, so oscillation or overshoot may occur. The voltage sensing portion of the volt time limit circuit also requires time to recover, so power train components may be overstressed due to excess current or volt time.
Following a line dropout it is desirable that the output voltage return to the correct level in a controlled manner as soon as possible without overshoot and without overstressing power train components. A commonly used partial solution to the problem is direct sensing of current by use of either a current sensing transformer or resistor. However, current transformers are large and costly, and resistor sensing is dissipative and prone to noise problems.
It is an object of the present invention to provide a control circuit that allows smooth recovery from momentary power line drop outs.